image texture on plane leaves average color of image in 3d-view
When using an Image Texture you need to UV unwrap your mesh.

To do this, go into edit mode, select all vertices, and press U > Unwrap.



What is happening, is Blender has no coordinates to map the texture to, so it takes the average image color and uses that as the texture. When you UV unwrap the mesh, you are telling Blender what vertices go where in relation to the texture, and Blender can then project the texture onto the mesh.

If you now go to the UV/Image Editor you can see and edit how your mesh has been unwrapped: